A typical optical communication system includes a pair of network nodes connected through an optical waveguide (for example, an optical fiber). An optical signal is transmitted between each pair of nodes through an optical transmission link. The optical transmission link between the network nodes is generally constructed by multiple optical modules, and multiple optical fiber spans are connected through optical amplifiers. FIG. 1 shows a typical wavelength division multiplexing transmission system. Optical signal transmitters 1011˜101M generate M transmission signals of different wavelengths, and the signals are combined into a combined optical signal through a wavelength division multiplexer 102 and then transmitted through an optical fiber link. The transmission link includes N optical fiber spans 1031˜103N that are connected through optical amplifiers 1041˜404N, where D1˜DN are dispersion caused by optical fiber spans. At a receiving end, a wavelength division multiplexing signal first passes through a demultiplexer 105 to separate signals of different wavelengths, and the signals are received by receivers 1061˜106M respectively to restore original information.
During a process in which the optical signal is transmitted through the transmission link, the damage such as dispersion, polarization mode dispersion, polarization dependent loss, ray nonlinearity, and noise of amplifiers is caused, so that the performance is deteriorated. In the current optical communication network, generally, it is expected to increase the incident power of a transmission signal, so that the signal received at the receiving end has power that is large enough, so as to ensure that the receiving end has a signal to noise ratio that is large enough after the signal passes through the transmission link, and ensure that a code error rate of the receiving end is less than a certain threshold, thereby enabling the signal to be transmitted effectively. However, due to the nonlinear property of optical fiber transmission, the increasing of the incident power amplifies the nonlinear effect during the transmission, and therefore, the increasing of the incident power cannot always increase the transmission performance. If the incident power is already greater than the optimal incident power (the system performance is optimal when the incident power is the optimal incident power), the increasing of the incident power results in the reduction of the system performance. Therefore, in prior art, generally, the nonlinearity of the optical fiber is compensated to expand the possible transmission distance, so as to improve the performance of the system.
FIG. 2-a is a schematic structural diagram of performing compensation on an optical signal at a receiving end in a single polarization system in the manner of digital signal processing. A received optical signal 201 and a local optical carrier signal generated by a local laser 202 are input to a coherent receiver front-end 203. The coherent receiver front-end 203 converts the received optical signals into in-phase and quadrature (that is, I/Q) baseband electrical signals, which pass through an analog-to-digital converter A/D 204 that outputs sampled digital signals, and the digital signals are input to a compensation module 205 to complete dispersion compensation and nonlinear compensation. The dispersion compensation and nonlinear compensation are accomplished in series by adopting N compensation modules. Each compensation module includes a dispersion compensation module 205ia and a nonlinear compensation module 205ib (i is any value between 1 and N) that are connected in series. The compensated signals are input to a self-adaptive equalization module 206, so as to compensate residual system damage and track system changes. An output signal of the self-adaptive equalization module 206 is input to a phase restoration module 207 to compensate phase noise caused by frequency difference line-width of the laser, and a determination module 208 performs determination to restore an original bit sequence. FIG. 2-b is a specific implementation of the compensation on signal by nonlinear compensation module 2051. The dispersion compensation module 205ia is implemented through a frequency domain, that is, an input signal A is first converted into a frequency domain signal through Fast Fourier Transformation (FFT, Fast Fourier Transformation) and multiplied by an ith function HCD-i of frequency domain dispersion compensation, the frequency domain signal is then converted into a time domain signal B through Inverse Fast Fourier Transformation (IFFT, Inverse Fast Fourier Transformation), and the time domain signal B is sent to the nonlinear compensation module 205ib to accomplish the nonlinear compensation shown in the drawing. The compensated dispersion is dispersion caused by an (N−i+1)th optical fiber span in the transmission link, that is, in the modules 2051˜205N, the dispersion compensated by the dispersion compensation modules 2051a˜205Na is the dispersion caused in DN˜D1 in FIG. 1, respectively.
It can be seen from the nonlinear compensation module 205ib that, the foregoing solution performs the nonlinear compensation by adopting a formula B*exp(−jγ|B|2), B is obtained on the basis of a dynamic received data stream, and therefore, in the foregoing solution, a sine function lookup table and a cosine function lookup table related to B are first calculated, for example, values of cos γ|B|2 and sin γ|B|2 corresponding to different Bs, so that a result of B*exp(−jγ|B|2) is calculated through looking up the tables when the nonlinear compensation is performed. The foregoing solution requires calculation through looking up tables when the nonlinear compensation is performed, resulting in a long delay of the processing.